In small loudspeaker enclosures (e.g. diameter of 50 mm to 64 mm), such as those designed for telephone sets, fairly deep nulls occur at mid to high frequencies due to cavity modes in the enclosure. Because inexpensive components are normally used in the construction of such enclosures, cost constraints generally prohibit modification of the loudspeaker characteristics, such as by damping. In order to obtain high efficiency and the lowest f0 possible, the diaphragm of such small loudspeakers is generally not very well damped. The diaphragm is therefore sensitive to the acoustic resonance of the enclosure cavity, which effectively ‘blocks’ the diaphragm and results in strong notches in the frequency response curve, often occurring in the frequency band of interest.
It is known in the art to provide optimal porting of the loudspeaker enclosure to modify the loudspeaker frequency response. For example, porting of loudspeaker enclosures has been used extensively for extending bass response (see U.S. Pat. No. 1,869,178 (Thuras)). Leo L. Beranek, in Acoustics, Acoustical Society of America 1996 (reprint of 1954 text), provides a very clear description of the basic assumptions and physics in designing a ported loudspeaker enclosure. The primary assumption made is that for low frequencies the wavelength of interest is large compared to the enclosure dimensions, and that the effect of the port is negligible (i.e. the port impedance becomes very large) at higher frequencies. An electrical (or mobility) analogy, known as ‘lumped parameter’, is derived making the shape of the enclosure and location of the loudspeaker, port, tube, and damping inconsequential.
Since the patent of Thuras, a large number of additional patents have issued describing inventions for correcting many of the problems encountered in specific and in general applications of ported enclosures, as set forth in greater detail below. It will be noted that each of these prior art patents is concerned only with the low frequency performance of the systems and that, because of the assumptions made for the lumped parameter modelling, the actual position of the port is not critical. Colloms suggests that, for small enclosures “it is more common to locate the exit facing away from the listener to reduce the audibility of the unwanted sounds, duct blowing and resonances and acoustic leakage from within the enclosure” (see Martin Colloms, High Performance Loudspeakers 5th ed., John Wiley & Sons, 1999).
The use of the lumped parameter method for loudspeaker modelling using electrical components has led to the recognition that the use of multiple ports can be beneficial. U.S. Pat. No. 4,549,631 (Bose) discloses a two port, two cavity loudspeaker while U.S. Pat. No. 5,714,721 (Gawronski) discloses a multi-chamber four port arrangement. U.S. Pat. No. 6,223,853 (Huon) presents the argument that the lumped parameter equivalents of the prior art limit themselves to the fundamental resonant frequency. Huon then presents a more complex model permitting the design of waveguides with at least two sections resulting in more accurate acoustical filters.
As alluded to above, a ported enclosure can exhibit resonant frequencies above those of interest. In U.S. Pat. No. 2,031,500, Olney discloses a folded duct that is lined with acoustically absorptive material so as to permit only low frequency sound to propagate and eventually emanate from the end of the duct. Olney claims that this reduces the “air cavity resonance effect.” U.S. Pat. No. 4,628,528 (Bose) uses substantially the same idea but purposely makes the duct as rigid as possible. The various tubes are arranged to provide significant gain (especially in the low frequencies). U.S. Pat. No. 6,278,789 (Potter) attenuates the high frequencies in such a waveguide by the use of a polyester baffle in the cavity placed close to the loudspeaker. U.S. Pat. No. 6,275,597 (Roozen) discloses the use of tuned resonators along the port tube to eliminate unwanted resonances.
As the loudspeaker is reduced in size, the performance of the loudspeaker becomes more demanding and the air velocity through the port becomes larger due to the smaller area. U.S. Pat. No. 5,757,946 (Van Schyndel) discloses the use of a ferro-magnetic fluid to improve the low frequency performance of a small loudspeaker. U.S. Pat. No. 5,517,573 (Polk) discloses a method to reduce the air turbulence noise that results from the use of small area ports.
In commonly-owned US Patent Application No. 2003/0063767, a cap is disclosed to control the effect of acoustic modes that ‘block’ the loudspeaker diaphragm displacements, thereby decreasing the sound pressure radiation thereby and creating large nulls in the frequency response.
It is an object of an aspect of the present invention to provide an acoustic enclosure with an aperture for providing a leak to correct cavity mode effects. As an added benefit, the aperture can be designed to serve as bass-reflex for low frequency enhancement.